Question tags are words that are said at the end of utterances in order to make a question. English has a very complex set of so-called “tag-questions” because of all the many auxiliary and modal verbs in the language (do, should, can, etc.). It is possible to also use the original verb in Portuguese to make a tag question, as in Você visitou os Estados Unidos, não visitou? (You visited the United States, didn't you?). However, it is far more simple (and more common) to simply use the expression não é? at the end of any declarative sentence to make a question. So, for example, you could say Ele não comeu nada, não é? (He did not eat anything, did he?).
In Brazilian Portuguese, speakers will very often reduce the expression não é? to the one-syllable né? Common sentences would be Ela vai ligar depois, né? (She will call later, won't she?), and so forth. Pay attention to this frequent abbreviation and use it yourself sometimes!